This invention is generally directed to improvements in television receivers. It is particularly directed to an AGC (Automatic Gain Control) system for selectively controlling the gains of UHF and VHF tuners.
Conventional television receivers include a VHF tuner for reception of 12 television channels in the very high frequency band and a UHF tuner for reception of additional channels in the ultra high frequency band. The outputs of these tuners are converted to an IF (intermediate frequency) signal by means of a mixer whose output is applied to one or more stages of IF amplication. A detector normally converts the amplified IF signals to baseband, and an AGC control senses the strength of the detector's output. Any deviation from nominal in the detector's output results in the AGC control developing a control signal which is used to correct the gain of the IF amplifier or the appropriate tuner.
In addition to holding the detector's output constant, one of the objectives of an AGC system is to obtain an undistorted, relatively noise free signal at the output of the detector. As is well known, the noise which appears at this point in the receiver depends on the strength of the signal received by the tuner, the characteristics of the circuitry following the tuner, and the noise figure and other characteristics of the tuner. Thus, to obtain a relatively clean signal at the detector's output, the AGC action should at least take into account the noise contributions associated with the tuner.
To obtain a detected signal which is relatively noise free, it is common practice to design the AGC control so that gain reduction is applied first to the IF amplifiers. At a certain higher strength of the incoming television signal, the gain of the IF amplifiers is held constant and the gain of the tuner's RF amplifier is reduced so as to avoid overloading the mixer. The cross-over point at which gain reduction is switched from the IF amplifier to the tuner is sometimes referred to as the AGC delay point.
The choice of an AGC delay point is usually determined by the performance characteristics of the VHF tuner, and that point is normally set by a potentiometer, a voltage divider, or the like. Once the delay point has been selected, it is used as the starting point for gain reduction of both the VHF tuner as well as the UHF tuner. This selection criteria can result in a detected signal which is too noisy when the UHF tuner is operative, primarily because UHF tuners have inherently poorer signal-to-noise characteristics than VHF tuners. In addition, the AGC delay point is sometimes compromised to avoid so-called "channel 6" beats from the mixer during VHF reception. This compromise causes the tuner's gain reduction to begin even sooner, and further deteriorates the noise condition of detected UHF signals. Hence, even under relatively strong signal conditions, a UHF television image may appear somewhat noisy.
The prior art has suggested using separate AGC voltages to control the gain reduction of the VHF and UHF tuners. See U.S. Pat. No. 3,688,198 for example.
Other suggestions from prior art include varying the point where the AGC becomes effective in an attempt to provide a constant signal-to-noise ratio, irrespective of the strength of the incoming signal. This proposal appears in U.S. Pat. No. 3,609,234.
The problem with both suggested approaches is that the gain reduction of the UHF and VHF tuners still begins at about the same point. Hence, the characteristics of the UHF tuner which render it inherently noisier are still not fully compensated for.